If you do a search on "pizza stone", you'll pull up several threads. The general gist is make sure you let the stone heat up with the grill. If you throw a cold stone on a hot grill, it will crack. Some folks use unglazed tiles or kiln bricks as well. I haven't tried it yet, but I have a stone in my cupboard just waiting for me to get around to it.

The wife and I just tried the grilled pizza. After reading the comments here about it, it just seemed like the thing to do for a Sunday dinner. I also sent a fair sized sample over to friends accross the street. (They are convinced I should open a restaurant after trying some samples of what I make from HTG)

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